Photovoltaic power generation (also referred to as “photovoltaics”) is a technology for generating electricity from solar energy. This technology directly converts solar energy into electric energy using a photoelectric converter that is referred to as a “solar cell”. Since the photovoltaic power generation can use limited amounts of light, it can also be used in cloudy weather and its utilization is higher than that of thermal power generation.
A variety of methods have been employed to achieve a reduction in the capacity of a photovoltaic power conditioning system and also to increase the efficiency of the system. One method is to reduce the number of components in the power circuit.
FIG. 1 illustrates an exemplary topology of PCSs for photovoltaic power generation. The conventional topology is mainly classified into an isolated PCS topology as shown in FIG. 2 and a non-isolated PCS topology as shown in FIG. 3. This classification is based on whether or not the solar array and the PCS output are isolated from each other with a transformer therebetween.
FIG. 1 is a block diagram of a conventional photovoltaic power conditioning system (PCS) employing an exemplary topology.
This system basically includes a DC/AC inverter 11, a sine filter 12, and a low frequency transformer 13. The DC/AC inverter 11 receives a DC voltage from a solar cell and converts it into an AC voltage. The sine filter 12 performs sine filtering on the AC voltage output from the DC/AC inverter 11. The low frequency transformer 13 converts the sine-filtered AC voltage into a low frequency and then outputs it to a general load.
Due to the characteristics of the solar cell, the output voltage of the solar cell varies between the maximum level and half thereof depending on the temperature and the level of irradiation. The photovoltaic power generation system is divided into a stand-alone type and a grid-connected type. The stand-alone type operates independent of the electric utility grid to supply AC voltage to a general load and the grid-connected type operates in conjunction with the grid to supply AC voltage to a general load. For example, a single-phase 220V grid-connected PCS is designed with a range of solar cell output levels of 150-400V. To supply 220V AC power with the wide range of input levels, the output voltage of the DC/AC inverter 11 is set to a voltage which it can generate with the minimum input voltage and the low frequency transformer 13 of the grid frequency is used to increase the output voltage in order to match it to the target AC output voltage “220V”.
One advantage of the topology of FIG. 1 is simplicity.
However, the system of FIG. 1 is big and heavy since it uses the low frequency transformer 13. In addition, since the output AC voltage of the DC/AC inverter 11 is generated based on the minimum DC input voltage, its current is increased for the same output power due to the low voltage. This increases the current capacity of components used for the DC/AC inverter 11. This also increases current flowing through the primary side of the low frequency transformer 13. This leads to an increase in the costs of the components and a reduction in the efficiency of the system.
FIG. 2 is a block diagram of a conventional isolation type photovoltaic power conditioning system. This topology overcomes the problems of the topology of FIG. 1 using an isolated DC/DC converter 21 having a high frequency transformer.
In the system of FIG. 2, the isolated DC/DC converter 21 having the high frequency transformer receives a DC voltage from a solar cell and converts it into another DC voltage and then performs high frequency transform of the converted DC voltage. A DC/AC inverter 22 converts the DC voltage output from the isolated DC/DC converter 21 into an AC voltage. A sine filter 23 performs sine filtering on the AC voltage output from the DC/AC inverter 22 and outputs the filtered AC voltage to a general load.
Since it uses the high frequency transformer, the system of FIG. 2 is advantageous over the system employing the low frequency transformer in terms of costs, size, and weight. In addition, the system of FIG. 2 can achieve a reduction in the current capacity of power components since it can keep the input voltage of the DC/AC inverter 22 at as high level as desired and can directly generate the output AC voltage.
However, the topology of FIG. 2 has the following problem. The efficiency of the system is expressed by the product of the efficiency of the converter at a front stage and the efficiency of the inverter at a subsequent stage. Without considering the efficiency of the common inverter, the efficiency of the converter must be at least equal to the low frequency transformer of FIG. 1 to allow the total efficiency to be equal to or higher than that of the system of FIG. 1. However, the efficiency of the converter is generally lower than the low frequency transformer, so that the efficiency of the system is lower than that of FIG. 1.
FIG. 3 is a block diagram of a conventional non-isolation type photovoltaic power conditioning system.
This system includes a non-isolated DC/DC converter 31, a DC/AC inverter 32, and a sine filter 33. The non-isolated DC/DC converter 31 receives a DC voltage from a solar cell and converts it into another DC voltage. The DC/AC inverter 32 converts the DC voltage output from the non-isolated DC/DC converter 31 into an AC voltage. The sine filter 33 performs sine filtering on the AC voltage output from the DC/AC inverter 32 and outputs the filtered AC voltage to a general load.
FIG. 3 shows a PCS topology using a non-isolated converter. Since circuitry of the non-isolated converter is simpler than that of the isolated converter, the non-isolation type PCS is advantageous over the isolation type PCS in term of price and thus the non-isolation type PCS has recently attracted a lot of attention.
However, the topology of FIG. 3 has the same problem as that of FIG. 2. Moreover, all the conventional topologies are similar in that the capacity of the converter required by the system is the same as the total capacity of the system. For this reason, the converter cannot be applied to a large capacity system despite a lot of advantages and as such can be applied only to a small capacity system.
In the exemplary topology of FIG. 1, the capacity of the converter must be equal to that of the system since it does not fully utilize the output characteristics of the solar cell, thereby reducing the efficiency of the system.